The present invention relates generally to methods for making tissue products. More particularly, the invention concerns methods for making tissue having high bulk and absorbency on a modified conventional wet-pressing machine.
In the art of tissue making, large steam-filled cylinders known as Yankee dryers are commonly used to dry a tissue web that is pressed onto the dryer cylinder surface while the tissue web is still wet. In conventional tissue making, the wet paper web is firmly pressed against the surface of the Yankee dryer. The compression of the wet web against the dryer surface provides intimate contact for rapid heat transfer into the web. As the web dries, adhesive bonds form between the surface of the Yankee dryer and the tissue web, often promoted by sprayed-on adhesive applied before the point of contact between the wet web and the dryer surface. The adhesive bonds are broken when the flat, dry web is scraped off the dryer surface by a creping blade, which imparts a fine, soft texture to the web, increases bulk, and breaks many fiber bonds for improved softness and reduced stiffness.
Traditional creping suffers from several drawbacks. Because the sheet is pressed flat against the Yankee, the hydrogen bonds that develop as the web dries are formed between the fibers in a flat, dense state. Although creping imparts many kinks and deformations in the fibers and adds bulk, when the creped sheet is wetted, the kinks and deformations relax as the fibers swell. As a result, the web tends to return to the flat state set when the hydrogen bonds were formed. Thus, a creped sheet tends to collapse in thickness and expand laterally in the machine direction upon wetting, often becoming wrinkled in the process if some parts of the laterally expanding web are restrained, still dry, or held against another surface by surface tension forces.
Further, creping limits the texture and bulk that can be imparted to the web. Relatively little can be done with the conventional operation of Yankees to produce a highly textured web such as the throughdried webs that are produced on textured throughdrying fabrics. The flat, dense structure of the web upon the Yankee sharply limits what can be achieved in terms of the subsequent structure of the product coming off the Yankee.
The foregoing and other drawbacks of traditional creping may be avoided by producing an uncreped throughdried tissue web. Such webs may be produced with a bulky three-dimensional structure rather than being flat and dense, thereby providing good wet resiliency. It is known, however, that uncreped tissue often tends to be stiff and lacks the softness of creped products. Additionally, throughdried webs sometimes suffer from pinholes in the web due to the flow of air through the web to achieve full dryness. Moreover, most of the world's paper machines use conventional Yankee dryers and tissue manufacturers are reluctant to accept the high cost of adding throughdrying technology or the higher operating costs associated with throughdrying.
Prior attempts to make an uncreped sheet on a drum dryer or Yankee have included wrapping the sheet around the dryer. For example, cylinder dryers have long been used for heavier grades of paper. In conventional cylinder drying, the paper web is carried by dryer fabrics which wrap the cylinder dryer to provide good contact and prevent sheet flutter. Unfortunately, such wrapping configurations are not practical for converting a modern creped tissue machine into an uncreped tissue machine. Moreover, without creping, the web may be stiff and have low internal bulk (low pore space between fibers). Further, high speed operation may not be possible due to impaired heat transfer. When a web is not heavily pressed into a flat state against the Yankee or drum dryer surface, conductive heat transfer is reduced and the drying rate is cut substantially. Another problem encountered at high speed is the difficulty of removing a web from a fabric to place it on the Yankee, especially if the fabric is highly textured or three-dimensional. The web often becomes firmly attached to the fabric, and the process of transferring the web from the fabric to the Yankee may cause picking of the web or other signs of undesirable sheet disruption or failure. Additionally, at commercial speeds, the problem of attaching and removing an uncreped, textured sheet from a Yankee surface is exceedingly difficult, as described hereinafter.
Prior tissue manufacturing methods have also employed rush transfer or negative draw of a wet sheet to improve the flexibility and softness of an uncreped, noncompressively dried sheet. The combination of rush transfer, web molding into a three-dimensional fabric, and drum drying, however, especially when operated without creping at industrially useful speeds, leads to several problems in practice which have not previously been recognized or solved. In particular, Applicants have discovered that the most highly stressed portions of the rush transferred sheet, when pressed onto the Yankee surface for drying, may fail or remain adhered to the Yankee when the sheet is removed with or without creping. The problem can be most harmful in uncreped operation because portions of the sheet may stick to the Yankee without a crepe blade to effect good removal, but degradation of sheet quality will also occur with creped operation. The result may be a high number of sheet breaks or an acceptable product having low strength, nonuniform properties, and sheet defects.
Thus, there is a need for a tissue making operation that overcomes the above-referenced problems of sheet molding, drying, attachment, and release on a Yankee dryer. In particular, there is a need for a process which allows uncreped or lightly creped production of textured tissue on a drum dryer at industrially useful speeds with minimal sheet failures. Desirably, the tissue sheet resulting from such operation has a three-dimensional topography for high apparent bulk, a noncompressively dried structure for high inherent bulk (defined hereinafter) and softness, and low damage during attachment and release for high strength of the soft, absorbent sheet.